The prior art describes various molybdenum-containing raw materials. They differ only slightly in the molybdenum content and the gangue composition. However, the mass ratio of the molybdenum oxides is very different. There are molybdenum raw material qualities which contain up to 50% molybdenum dioxide (MoO2) and other sparingly soluble molybdenum suboxides, such as Mo4O11. These raw materials are to date used predominantly in the steel industry since the hydrometallurgical conversion of such molybdenum raw materials into high-quality molybdenum compounds and finally into molybdenum metal is not economically viable.
In the production of molybdenum and molybdenum compounds in the chemical industry, only those raw materials which have a very high molybdenum trioxide content (MoO3) and which are very readily soluble in aqueous media are currently used. In view of increasingly scarce high-quality raw materials and increasing raw material prices, any possibility of using alternative raw materials for molybdenum extraction which have an economic advantage should be investigated.
In most cases, molybdenite (MoS2) serves as a molybdenum source. Molybdenite is usually converted into molybdenum oxide by roasting under oxidizing conditions. The molybdenum oxides thus obtained, also referred to as technical molybdenum oxides, are then digested by known processes.
DE 2162065 describes a process for purifying and digesting roasted molybdenum concentrates, also referred to below as technical molybdenum oxides. The technical molybdenum oxide is treated with a nitric acid which contains ammonium nitrate (NH4OH). The content of free nitric acid results in a sufficient decrease in the impurities and the further digestion of the Mo compounds, so that, on subsequent dissolution in NH4OH or NaOH, a comparatively smaller amount of residue on dissolution having a comparatively lower molybdenum content is obtained than on dissolution of untreated molybdenum concentrate.
U.S. Pat. No. 4,525,331 describes using a nitric acid (HNO3) excess to firstly achieve better solubility of the impurities and secondly to oxidize residues of Mo(IV) into Mo(VI). The solid is then dissolved in NH4OH.
U.S. Pat. No. 4,596,701 describes digesting the molybdenum oxides with an aqueous solution of sulphuric acid, ammonium sulphate and ammonium peroxodisulphate in order better to dissolve the impurities. Furthermore, the peroxodisulphate also oxidizes Mo(IV) still present to Mo(VI), so that a better molybdenum yield is achieved in the subsequent dissolution step with NH4OH.
U.S. Pat. No. 4,046,852 describes another process for preparing high-purity alkali metal molybdate which involves mixing the roasted molybdenum concentrate in a first step with mineral acids, the mineral acids being used in a stoichiometric excess, based on the impurities. In a second step, the suspension thus formed is treated in an autoclave at between 150° C. and 350° C. in the presence of free oxygen, impurities going into solution and Mo suboxides being converted into MoO3. In the third step, the solid separated off is dissolved in alkali metal hydroxide.
DE 2345673 describes a process for the preparation of high-purity molybdenum oxide and ammonium molybdate, the molybdenum oxide concentrate first being treated with an aqueous ammonium hydroxide solution for producing a suspension. The suspension thus obtained is then digested in the presence of free oxygen which is sufficient to convert the predominant proportion of the molybdenum oxides into soluble ammonium molybdate compounds. The residue, which still has proportions of molybdenum, is again leached with an aqueous alkaline solution which contains sodium hypochloride, at pH 9 to pH 10, in order to dissolve the remaining proportions of molybdenum.
In the abovementioned processes, separating off the impurities on digestion of the roasted molybdenum concentrate is of primary importance. Excessive reagent or addition of oxidizing agents relates primarily to the amount of impurities detected in the molybdenum oxide. The increase in the Mo dissolution yield on digestion of molybdenum concentrate by oxidation of the Mo suboxides during the digestion process is mentioned as a side effect. Molybdenum suboxide is oxidized to MoO3 only in an amount which corresponds to the stoichiometry of the amount of oxidizing agent used.
With the use of free oxygen as an oxidizing agent, an autoclave is employed at at least 150° C. and a pressure of about 7 bar. In the case of digestion of molybdenum concentrates with NH4OH at atmospheric pressure in the presence of free oxygen, it is necessary to employ a plurality of steps in order to achieve a satisfactory molybdenum yield.
In all the abovementioned processes, starting materials are used which either contain a very high proportion of molybdenum trioxide (MoO3) soluble in aqueous media or which consist exclusively of a contaminated molybdenum trioxide.